1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic apparatus, electrostatic recording apparatus and the like, and more particularly, it relates to an image forming apparatus which can form an image with different toners and has a fixing device for fixing the image by fusing the toner.
2. Related Background Art
In a conventional image forming apparatus such as an electrophotographic apparatus, a latent image formed on a latent image bearing member was generally visualized by the use of a single kind of developer (referred to as "toner" hereinafter); however, with the advance of the recent diversity of the image forming techniques and/or colorization of the outputted image, various apparatuses for obtaining a full-color image or plural monocolor images have been proposed and applied in practical use. The image forming process in these apparatuses is fundamentally the same as that of the image forming apparatus utilizing a single toner. However, in these apparatuses, since a plurality of toners are used, a plurality of developing containers (as developing means) corresponding to the number of toners to be used are incorporated into the apparatus, or are provided for interchanging to obtain a desired color image. Further, normally, a fixing device comprises a rotary member heated by a heat source and a back-up rotary member, whereby the image is fixed by passing a recording medium having the toner image formed thereon through between these rotary members.
However, in the fixing device used with the above-mentioned conventional image forming apparatus, since the same fixing operation is effected with respect to various kinds of toners in spite of the fact that each toner has a different offset generating temperature, in accordance with the toner component such as resin, magnetic component of the like, there arises a problem that the toner is adhered to the fixing rollers to finally generate the offset phenomenon. For example, when both the magnetic toner including the magnetic component and the non-magnetic toner not including the magnetic component for obtaining the sharp colored image are used, since the non-magnetic toner has a lower offset generating temperature than that of the magnetic toner, if the magnetic toner is used as the black toner, the non-magnetic toner generates the offset in the fixing process.
In order to reduce or eliminate the offset toner, as shown in FIGS. 6A to 6C, a positive cleaning means has normally been provided in the fixing device. Incidentally, FIG. 6A shows an example that a cleaning blade is arranged to contact with the fixing roller, FIG. 6B shows an example that a felt oil pad impregnated with the silicone oil is arranged to contact with the surface of the fixing roller, and FIG. 6C shows an example that a web impregnated with the silicone oil is arranged to contact with the surface of the fixing roller.
However, even when the fixing roller is supplied with the silicone oil or is cleaned by the cleaning blade in this way, if the offset of the toner is too severe, a large amount of silicone oil must be applied onto the surface of the fixing roller, with the result that there arises a problem that the recording medium is smeared with the oil and/or the oil pad or web must frequently be exchanged or replaced.